django_on_call.models: Make OnCall.slug unique

Using the unique option [1].  Having multiple OnCall objects for the
same position/slug is a bad idea.

[1]: https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.Field.unique
django_on_call.models: Add OnCall.__str__ and .__repr__

This makes the list of model instances on the admin page more
readable; without __str__ you'll just see "OnCall object" for each
instance.